1) Do not play yourself. You're doing this partially to prove that you can act when given the opportunity, and part of acting is playing other people. Dwayne "The Rock" Johnson pretty much began his eventual movie career by offering this simple instruction for his first SNL hosting gig: "No wrestling sketches". He was willing to wear a tutu, he was willing to make a complete fool of himself, but he refused to basically play himself in any wrestling sketches. He's now one of the very best reoccurring hosts. Point being, do not do what's expected of you.
2) If at all possible, avoid the question-and answer opening monologue. Unless you're Christopher Walken, this isn't going to be funny. It will be badly costumed cast members asking the most obvious, unfunny questions leaving you to give lame, scripted answers. Once again, unless you're Chris Walken, you cannot do this and maintain dignity. And if you're forced to do this bit, please make sure that every question isn't some horny nerd propositioning you for sex. It won't be funny and it will show the usual lack of imagination that cripple these opening bits.
3) If you must do a Transformers sketch, wait till the second half of the show. Normally I'd say avoid a Transformers sketch in general. But Efron used his High School Musical character as a springboard for a bloody hysterical sketch about plummeting educational standards, so I can't veto the idea completely. But hold off any sketches based around Transformers or Jennifer's Body until after Weekend Update. If the show up to that point is funny, then the audience won't care about a one-off miss. If you're already tanking, then it won't matter either way.
4) It's your show. At the end of the day, you can't blame the writers if the hosting gig tanks. You have one advantage: no one expects you to be funny. Maybe it's because of the 'attractive women aren't funny' stereotype, or maybe it's because you're best known for your physical attributes, but you now have the same trump card that made hosting gigs by John Malkovich or Patrick Stewart into winners. Because of who you are, every joke that scores is now twice as funny because you're the one you made it. Make sure that the sketches are written around you, or that you're a supporting player in the ones that work. Don't be labeled a Nancy Kerrigan or Paris Hilton by having the sketches be written around you, giving you little to do and say. Be in a position to take the credit for a hit or a miss. And you're going to take some critical hits regardless, simply because pundits and gossip columnists won't want to admit it if the young actress that they spent all summer calling a whore and an idiot turns out to be intelligent and witty.
So good luck kid. I may hate your movies, I'm not all that attracted to you, and I have no idea if you can actually act, but your candor in interviews and the resulting blowback by the gossip world and entertainment press has earned my grudging respect. And, of course, if you can get Tina Fey to come back and participate in a sketch involving videotaped car washes, well, then you'll be my hero (and if you can get Seth Meyers to participate, my wife will highly approve).